1. Field of Arts
This invention relates to a process for producing an optically isotropic reformed pitch useful for various carbon materials by reacting a pitch in the presence of a strong Lewis acid and a co-solvent. Since the said reformed pitch has a characteristic property that it has a high fixed carbon content in spite of its low softening point and low quinoline insoluble content and that it can be converted easily to mesophase by heat treatment, the said reformed pitch can be used for various purposes, for example, an impregnant for high grade carbon materials such as carbon-carbon composite materials, artificial graphite electrodes, carbon graphite shaped articles, etc., a raw pitch for mesophase pitch based carbon fibers, and a mixing material for modifying various kinds of pitch.
This invention also relates to a process for producing spherical meso-carbon microbeads which comprises subjecting the said reformed pitch to a heat treatment at a temperature of 200.degree..about.380.degree. C. Further, this invention relates to a process for producing meso-carbon microbeads having a uniform diameter in the range of 0.5.about.20 .mu.m, at a high yield of about 60% or more.
Meso-carbon microbeads are spherical carbon materials having a structure in which highly condensed polycyclic aromatic hydrocarbons are arranged in a definite direction. Since meso-carbon microbeads have properties inherent to carbon chemically, electrically and magnetically and have a good sintering property during carbonization, they are used for various industrial materials such as an electro conductive filler, a binderless isotropic high density carbon material, a catalyst carrier a chromatogram filler, etc., in the form of meso-carbon microbeads as they are produced or after carbonization.
2. Prior Art
High grade carbon materials such as carbon-carbon composite materials, artificial graphite electrodes, carbon-graphite shaped articles, etc. are generally produced by shaping, carbonizing and graphitizing a mixture of a basic material such as shaped cokes and a binder pitch. In the case of producing a high density, high strength material, it is necessary to repeat pitch impregnation and carbonization process several times before graphitization. The impregnant is indispensable for producing a high grade carbon material, because the pitch impregnation of a carbonized material is effective to bind the basic materials with each other, to decrease porosity and increase density, strength, electric conductivity and thermal conductivity of the produced carbon material.
Pitch based impregnants are generally produced from petroleum or coal based pitch by heat treating to cause a condensation polymerization reaction and to remove a low boiling point fraction.
Pitch based impregnants are required to have properties as follows for various purposes.
(1) Low quinoline insoluble (QI) content PA1 (2) Low softening point PA1 (3) High fixed carbon content PA1 (4) High resin content (difference between benzene insoluble content and QI content) PA1 (5) Low ash content PA1 (6) Low content of low boiling point fraction
Among these properties, low QI, low softening point and high fixed carbon content are very important.
According to the conventional production process of pitch based impregnants, if the softening point were lowered to improve an impregnation efficiency, the fixed carbon content would extremely decrease, therefore, a pitch impregnation and carbonization process would have to be repeated several times. While if the fixed carbon content were raised to decrease the number of repetitions of pitch impregnation and carbonization, the impregnation efficiency would extremely decrease because of the increase of the QI content and the elevation of the softening point, and a solvent extraction process would be necessary to remove quinoline insoluble components.
According to the conventional production process of meso-carbon microbeads, the production of meso-carbon microbeads having a very small diameter, particularly 5 .mu.m or less has been difficult.
Further, there is a problem in that if it is intended to increase the yield of optically anisotropic small spheres, the small spheres coalesce and precipitate to produce bulk mesophase and separation of small spheres becomes difficult.
Some arts disclose the modification of product quality by coexistence of a Lewis acid in converting pitches to mesophase. Official gazette of patent publication Sho 53-7533 discloses a process for producing a mesophase pitch having a softening point of 200.degree..about.300.degree. C. which comprises directly adding a solid Lewis acid such as AlCl.sub.3 or the like to a petroleum based tar or pitch having a softening point of 120.degree. C. or lower, subjecting the resulting mixture to heat treatment at a temperature higher than the softening point of the said mixture, usually at 200.degree..about.300.degree. C., and after removing the catalyst, subjecting to the second heat treatment at a temperature of 350.degree..about.500.degree. C.
Since the flow characteristic of non-mesophase component is close to that of mesophase component in this mesophase pitch, spinnability is excellent even when mesophase content is low and it is said that this mesophase pitch is preferable as a raw material of carbon fibers. However, the temperature of the first heat treatment should be kept high to perfectly melt the solid Lewis acid. There is no disclosure about the production of meso-carbon microbeads.
In U.S. Pat. No. 4,457,828, there is disclosed a mesophase pitch having molecules of ellipsoidal shape which are produced by the polymerization of aromatic hydrocarbon containing two or more condensed rings. The molecules of this mesophase pitch have been polymerized as 60% or more of bonds which connects condensed rings are coupling (bonding which does not form ring closure) and have a long and slender shade as a whole and considered to be close to ellipsoid.
This polymerization reaction is carried out using a catalyst of a weak Lewis acid such as an anhydrous AlCl.sub.3 accompanied by a second component such as CuCl.sub.2 which has a function of reducing the activity of AlCl.sub.3. As a solvent, orthodichlorobenzene, nitro benzene and trichlorobenzene are preferable. It is said that the mesophase pitch which is obtained by subjecting a pitch, from which a catalyst has been removed, to heat treatment is preferable for spinning probably due to its slender molecule. Further, this mesophase pitch has a low softening point and good shaping property at a low temperature. It is said that this mesophase pitch has a thin laminate layer of molecule compared with conventional mesophase pitches in spite of high completeness of crystal formation. Further, it is said that this mesophase pitch has characteristic properties different from the mesophase pitch produced by using a strong Lewis acid which is not accompanied by the second component. According to this process, the control of the catalyst system is complicated and the growth of mesophase is restrained. There is no disclosure about the production of meso-carbon microbeads.
As for a process for producing meso-carbon microbeads, as disclosed in e.g. in official gazette of patent publication No. Sho 50-39633, a process in which petroleum based or coal based pitch is subjected to heat treatment at a temperature of 350.degree.-500.degree. C. at a relatively slow heating rate (10.degree. C./min. or less), has been heretofore carried out.
The problem of this process is in the difficulty of production of meso-carbon microbeads having uniform diameter with a high yield. Even when good quality pitch containing no free carbon was used as a raw material, yield was 10 vol % or less.
In the production of meso-carbon microbeads, it is considered that if the temperature in the inside of a reactor is made uniform, and by-product lower boiling point components are discharged efficiently to the outside of a system, long reaction time can be shortened. Official gazette of patent publication No. Sho 53-9599 discloses an art of producing meso-carbon microbeads in which a pitch in a reactor is heated by blowing superheated steam and vigorous stirring is carried out at the same time to produce optically anisotropic small spheres within a relatively short period of time and meso-carbon microbeads are produced from the said reaction product.
The problem of this art is that it is difficult to prevent the optically anisotropic small spheres from colliding and coagulating with each other. In this patent, the yield of optically anisotropic small spheres is suppressed at a low value a little over 10%. On this account, there is a problem that the amount of discarded pitch increase, resulting in high cost.
As for a process for producing meso-carbon microbeads having a uniform diameter, official gazette of patent publication Sho 59-17043 discloses heat treatment of pitch is carried out twice in order to obtain optically anisotropic small spheres. In this process, optically anisotropic small spheres which precipitates at the second heat treatment are removed and only floating optically anisotropic small spheres after the second heat treatment are picked up to make uniform the diameter of the small spheres and to acquire only small spheres having high degree of perfect circularity. It is said that the meso-carbon microbeads obtained according to this process are excellent in the quality such as uniformity of diameter and degree of perfect circularity and are convenient to be used. But, the yield of meso-carbon microbeads from a raw material pitch is still about 10% and high cost is a problem.
It is an object of the present invention to produce a low softening point, low QI content and high fixed carbon content optically isotropic reformed pitch useful for various carbon materials.
It is another object of the present invention to overcome the problem of conventional process for producing meso-carbon microbeads, i.e. low yield and high cost and to produce meso-carbon microbeads having a uniform diameter of 20 .mu.m or less with a high production yield of about 60% or more by suppressing the coalescence and precipitation of optically anisotropic small spheres during heat treatment of pitches.